Ozone that causes photochemical smog is generated as a result of HC and NOx that are contained in exhaust gas of automobiles and factories causing a photochemical reaction. Therefore, suppressing ozone generation by reducing emissions of HC and NOx from automobiles is an effective way of preventing the occurrence of photochemical smog. On the other hand, direct purification of ozone in atmospheric air is also a conceivable method for preventing the occurrence of photochemical smog. It is possible to prevent the occurrence of photochemical smog more effectively by not only aiming at reducing emissions of HC and NOx as reactants, but also trying to purify ozone that is a product. Against this background, automobiles provided with an atmosphere-cleaning device for vehicles that enables direct purification of ozone in atmospheric air have been put into practical use in some regions, such as California in the United States. The aforementioned atmosphere-cleaning device for vehicles is referred to, in particular, as a “direct ozone reduction (DOR) system”.
For instance, as a DOR system, Patent Literature 1 discloses a system in which a metal oxide such as manganese dioxide is carried by a vehicle component such as a radiator or an intercooler. The radiator or intercooler is installed at a location at which a flow path of atmospheric air is formed while the vehicle is travelling. The metal oxide such as manganese dioxide is an ozone purifying body having a function of purifying the ozone present in atmospheric air by converting the ozone into another substance such as oxygen. Therefore, according to the DOR system disclosed in Patent Literature 1, ozone present in atmospheric air can be directly purified while the vehicle is travelling.